Fertilizer salt index and "burn" potential

As with manure, some chemical fertilizers can injure or even kill seeds or plants when placed too close, and this is called fertilizer "burn". The likelihood of burn depends on the fertilizer used, its rate and placement, and the type of crop.

What Causes Fertilizer Burn?

Fertilizers are composed of various types of salts such as chlorides, sulfates, and nitrates. Some of these dissolve very readily in the soil water after application. If too high a salt concentration accumulates near the seed or roots, they become unable to absorb enough moisture and show many of the symptoms of drought. If you've taken a biology course, you may remember the principle of osmosis, which seeks to equalize the salt concentration of 2 solutions separated by a permeable membrane (in this case, the seed coat or root hair surface). This is what causes most fertilizer burn. The difference in salt concentration between the inside of the seed or roots and the soil water outside them creates an osmotic "pull" that either prevents water from being taken in or actually draws it out of the plant tissues.

How to Spot Fertilizer Burn

Here are some symptoms that can indicate fertilizer burn.

• Poor seed germination (poor seedling emergence): However, this can be caused by many other factors like low-viability seed, disease, lack of soil moisture, etc.

• Seedlings begin to wilt, and then become yellow and eventually brown and dead, starting at the leaf tips. This can also be caused by other factors such as drought, insects, and diseases. Even established plants can suffer fertilizer burn if a nitrogen sidedressing is applied too close or at too high a rate.

Other Types of Fertilizer Burn

• Some fertilizers like urea and all-ammonium phosphate can also cause burning by releasing free ammonia gas if placed too near seeds, seedlings, or established plants.

• Fertilizer granules containing N and K will cause burn spots on plant leaves if spilled on them.

• Foliar applications will burn the leaves if too strong.

Fertilizers Vary in their Burn Potential

Fertilizers vary in their soluble salt contents. Those containing N and K have the highest salt ratings and are much more likely to cause burn than straight P fertilizers like superphosphate. Some fertilizers like urea and all-ammonium phosphate release free ammonia gas which can also cause burn when placed too close.

TABLE 9-2 Relative Burn Potential of Common Fertilizers

How to Prevent Fertilizer Burn

As shown by Table 9-2, those fertilizers containing N and K are much more likely to cause burn than those that contain only P, such as superphosphate.

Fertilizer burn is more likely when a localized placement method is used (band, hole, half circle). Follow the distance guidelines carefully that are given in the fertilizer application methods section farther along.

Fertilizer burn is more likely to occur on sandy soils, since salts and free ammonia move more readily.

• Fertilizer burn is gore likely under low-moisture conditions.

• Avoid placing fertilizer in contact with seeds or directly under the seed furrow, even if separated by a few centimeters of soil. Salts can move upward an the soil dries out and can reach the seeds. However, superphosphate can be banded directly under the seed furrow if separated by soil.

• If sidedressing plants by broadcasting an N fertilizer, avoid applying it when the leaves are wet or else the granules may cling to the leaves and cause burn spots.

• If applying an N sidedressing by hand-watering plants with N fertilizer dissolved in water, be sure to wash off the leaves afterwards with plain water.

How to Treat Fertilizer Burn: If water is available, use liberal amounts to flush away the salts from the needs or seedlings. If not, hope for rain!